H A F L E R C I R C U I T
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I used the four-speaker variant of this Hafler circuit for 18 years
to play stereo records. It is roughly equivalent to a Dolby Surround
decoder and works well for Dolby MP encoded sources (I am told). It
does not compare with an Ambisonic Surround Sound decoder, but is a
great deal cheaper.
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Right + -----------------------------------------------
| |
P + +
Right front Right rear
O A - -
--- - Centre front + --- | |
W M Common - ---| |---+--- x Pot x ---|
---- x Dummy load x ---- | |
E P - -
Left front Left rear
R + +
| |
Left + -----------------------------------------------
Notes: 1) This circuit puts strange loads on your power-amp.
Use at your own risk.
2) For only four speakers, replace the centre front
speaker and dummy load with a short circuit.
3) The dummy load reduces the volume of the centre front
speaker. Theory suggests 19 ohms for 8 ohm speakers.
4) The potentiometer (Pot) needs to be wire wound to
take the power. I used 50 ohms with 8 ohm speakers.
5) To "decode" stereo sources I found a pot setting of about
25 ohms was best. There is no best setting for mono.
6) To decode Dolby MP sources set the pot to maximum,
omit it, or put it in series with the rear speakers.
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The circuit works like this. If you set the pot to zero, the rear
speakers are fed the same signal as the input. If you set the pot
to a high value (or omit it), the rear speakers are fed the pure
difference between the two input channels. For stereo, you will
want to set it to an intermediate value to blend a proportion of the
input with the pure difference into the rear speakers; I set mine to
about 25 ohms with 8 ohm speakers. The "correct" setting will be
slightly different for different stereo recordings, but you will
soon get tired of varying it and settle on a single compromise
setting. For mono recordings there is no "correct" setting. The
centre-front speaker is always fed the sum of the two input channels.
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A blend pot being connected to ground was first described by the late
Micheal Gerzon in 1971. You can download his article "A year of
surround-sound" from the Gerzon Archive:
http://www.audiosignal.co.uk/Gerzon%20archive.html
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Have fun,
Martin Leese
E-mail: please@see.Web.for.e-mail.INVALID
Web: http://members.tripod.com/martin_leese/
19 February 1997
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